JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>Item</li>
    <li>item</li>
    <li>item</li>
    <li>item</li>
    <li>item</li>
</ul>

CSS

ul{
    list-style: none;
    width: 120px;
    margin: 50px;
}
li {
    margin: 8px 0 0 0;
    width: 100px;
    background: grey;
}
li:hover:before{
    content: "";
    position: absolute;
    width: 0;
    height: 0;
    border-top: 10px solid transparent; /* Рисуем треугольник */
    border-right: 10px solid red;
    border-bottom: 10px solid transparent;
    margin-left: -10px;
}

li:hover {
	background: red;
}